Problems with Multiple VPN Passthrough

we are 4 people in the office simultaneously using the XP (SP2) VPN clients to connect a remote VPN server (4 VPN pass through at the same time). The local router is a WRV54G with latest firmware (2.37). some stations are wired some are wireless.

After a while (minustes or hours) the VPN remote gateway doesn't repond to ping command (ping timeout) and it becomes really difficult to restart/reconnect the VPN client (error 800). If i restart the local router it solves the issue for the next couple of hours. The internet connection always works.

Here is the config
- router has static IP with direct internet connection (no other modem or equipement)
- IPSec, L2TP and PPTP Passthrough are all enabled.
- the router MTU is set to 1398 so that packet are not fragmented when accessing the remote VPN gateway.
- Port trigerring is enabled for port 500 and 1723.
- XP registries are modified according to microsoft KB885407 article: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\IPSec
"AssumeUDPEncapsulationContextOnSendRule"=dword:00000002

I had a look at the router logs and found several warning/errors:
alg_nat_states_add: Failed to add NAT state
alg_nat_states_add: Failed to add NAT state
pptp_gre_connect() Couldn't add GRE nat
alg_nat_states_add: Failed to add NAT state
alg_nat_states_add: Failed to add NAT state